Diodes AP2301/AP2311 - Manual (Page 10)

AP2301/AP2311
Document number: DS32241 Rev. 5 - 2
10 of 18
www.diodes.com
February 2014
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AP2301/AP2311
Application Information
Power Supply Considerations
A 0.1μF to 2.2μF X7R or X5R ceramic bypass capacitor placed between IN and GND, close to the device, is recommended. When an external
power supply is used, or an additional ferrite bead is added to the input, high inrush current may cause voltage spikes higher than the device
maximum input rating during short circuit condition. In this case a 2.2μF or bigger capacitor is recommended. Placing a high-value electrolytic
capacitor on the input and output pin(s) is recommended when the output load is heavy. This precaution reduces power-supply transients that may
cause ringing on the input. Additionally, bypassing the output with a 0.1μF to 1.0μF ceramic capacitor improves the immunity of the device to short
circuit transients.
Over-Current and Short Circuit Protection
An internal sensing FET is employed to check for over-current conditions. Unlike current-sense resistors, sense FETs do not increase the series
resistance of the current path. When an over-current condition is detected, the device maintains a constant output current and reduces the output
voltage accordingly. Complete shutdown occurs only if the fault stays long enough to activate thermal limiting.
Three possible overload conditions can occur. In the first condition, the output has been shorted to GND before the device is enabled or before V
IN
has been applied. The AP2301/AP2311 senses the short circuit and immediately clamps output current to a certain safe level namely I
LIMIT
.
In the second condition, an output short or an overload occurs while the device is enabled. At the instance the overload occurs, higher inrush
current may flow for a very short period of time before the current limit function can react. The input capacitor(s) rapidly discharge through the
device, activating current limit circuitry. Protection is achieved by momentarily opening the P-MOS high-side power switch and then gradually
turning it on. After the current limit function has tripped (reached the over-current trip threshold), the device switches into current limiting mode and
the current is clamped at I
LIMIT
.
In the third condition, the load has been gradually increased beyond the recommended operating current. The current is permitted to rise until the
current-limit threshold (I
TRIG
) is reached or until the thermal limit of the device is exceeded. The AP2301/AP2311 is capable of delivering current up
to the current-limit threshold without damaging the device. Once the threshold has been reached, the device switches into its current limiting mode
and is set at I
LIMIT
.
FLG Response
When an over-current or over-temperature shutdown condition is encountered, the FLG open-drain output goes active low after a nominal 7-ms
deglitch timeout. The FLG output remains low until both over-current and over-temperature conditions are removed. Connecting a heavy capacitive
load to the output of the device can cause a momentary over-current condition, which does not trigger the FLG due to the 7-ms deglitch timeout.
The AP2301/AP2311 is designed to eliminate false over-current reporting without the need of external components to remove unwanted pulses.
Power Dissipation and Junction Temperature
The low on-resistance of the internal MOSFET allows the small surface-mount packages to pass large current. Using the maximum operating
ambient temperature (T
A
) and R
DS(ON)
, the power dissipation can be calculated by:
P
D
= R
DS(ON)
× I 2
Finally, calculate the junction temperature:
T
J
= PD x R
θJA
+ T
A
Where:
T
A
= Ambient temperature °C
R
θJA
= Thermal resistance
P
D
= Total power dissipation
Thermal Protection
Thermal protection prevents the IC from damage when heavy-overload or short-circuit faults are present for extended periods of time. The
AP2301/AP2311 implements a thermal sensing to monitor the operating junction temperature of the power distribution switch. Once the die
temperature rises to approximately 140°C due to excessive power dissipation in an over-current or short-circuit condition the internal thermal sense
circuitry turns the power switch off, thus preventing the power switch from damage. Hysteresis is built into the thermal sense circuit allowing the
device to cool down approximately 20°C before the switch turns back on. The switch continues to cycle in this manner until the load fault or input
power is removed. The FLG open-drain output is asserted when an over-temperature shutdown or over-current occurs with 7-ms deglitch.
Under-Voltage Lockout (UVLO)
Under-voltage lockout function (UVLO) keeps the internal power switch from being turned on until the power supply has reached at least 2V, even if
the switch is enabled. Whenever the input voltage falls below approximately 2V, the power switch is quickly turned off. This facilitates the design of
hot-insertion systems where it is not possible to turn off the power switch before input power is removed.